Many types of information and content are now stored digitally, including books, music, movies, software programs, video games, databases, advertisements, as well as other content. Because such content is stored digitally, it can be transferred and sold easily using many types of electronic networks. See, for example, U.S. Pat. Nos. 5,875,110, 4,412,292, 5,848,398, 6,397,189, 6,381,575, 4,674,055, 5,445,295, 5,734,719, 6,286,029, 6,799,165, 6,655,580, 6,330,490, 6,662,080, 6,535,791, 6,711,464, 5,237,157, 6,654,757, 5,794,217, and 6,748,539. Both private and public electronic networks, including the internet, are frequently used to transfer the digital content.
But the digitization of content has also presented content providers, whether publishers or owners, with challenging problems. Digital content is often easy to copy and easy to distribute after it is sold. This situation makes unauthorized distribution of the digital content a concern for content providers because the unauthorized distribution detracts both from the revenue received from, and the control over, the digital content.
Current attempts at controlling the unauthorized copying and distribution of digital content include the use of digital watermarks. Digital watermarking is the process of altering the original data file by adding hidden data or information (i.e., copyright notices or verification messages) to the digital content without the user's knowledge. Such hidden data often contains information pertaining to the digital content or to the author of the digital content that can be used to either restrict the use of the digital content or as a mechanism to track the use of the digital content. Anyone with knowledge of the watermark and how it can be recovered can determine to some extent whether or not significant changes occurred to the digital content. With digital watermarks, the digital content cannot be typically altered without sacrificing the quality or utility of the digital content itself.